esbuild's default `esnext` target doesn't include `useDefineForClassFields: true`, unlike TypeScript

```
  --target=...          Environment target (e.g. es2017, chrome58, firefox57,
                        safari11, edge16, node10, ie9, opera45, default esnext)
```

The above is not true: the *default* is *NOT* `esnext`.

Found this while debugging #2992 